Extracorporeal blood treatment is a therapy that is widely used for critically ill patients. Many of these patients suffer from acute renal failure and are treated with continuous renal-replacement therapy (CRRT), a form of extracorporeal blood treatment that is normally performed in the Intensive Care Units (ICU's). In ICUs, CRRT therapy is mostly employed as so-called continuous veno-venous hemofiltration (CVVH) and to a lesser extent as continuous arterio-venous hemofiltration (CAVH) or continuous veno-arterial hemofiltration (CVAH), all of which represent various forms of hemofiltration.
Another form of renal replacement therapy that can be used for patients with renal failure in ICU's is hemodialysis. Pure hemofiltration as a renal-replacement therapy in an ICU can also be combined with hemodialysis as so-called continuous-veno-venous-hemodiafiltration (usually abbreviated as CVVHD or CVVHDF) or as continuous-arterio-venous-hemodiafiltration (usually abbreviated as CAVHD or CAVHDF). The addition of hemodialysis to a hemofiltration therapy implies the addition of a hemodialysis fluid (a so-called ‘dialysate’) flow, making such combined therapy forms more complex than pure hemofiltration. Hemodialysis usually can only be applied for a few hours per day and is much less effective than pure hemofiltration.
Typically, in extracorporeal treatments such as CVVH, CAVH, CVVHD, CAVHD, CVAH, and hemodialysis an artificial kidney is used. This kidney may be formed of hollow-fibers or of plates, and is connected to a patient's bloodstream by an extracorporeal circuit. In CVVH(D) the supply from and return to the blood of the patient is made via two venous accesses, using a blood pump to provide the driving force for the transport of blood from the patient into the artificial kidney and back to the patient. In CAVH(D), the access which provides the supply of blood to the artificial kidney is made via an artery and the return of the blood to the patient is made via a venous access. In this set-up in most cases blood pumps are generally not used because the arterial blood pressure is used to provide the driving force for the transport of blood, which implies that the blood flow rate directly varies with the blood pressure. Because of better control of blood flow, no risk of arterial catheter-related complications, and higher treatment efficiency, CVVH is preferred above CAVH as renal replacement therapy in ICU's.
In CVVH the patient's blood is passed through the artificial kidney, over a semipermeable membrane. The semipermeable membrane selectively allows plasma water and matter in the blood to cross the membrane from the blood compartment into the filtrate compartment, mimicking the natural filtering function of a kidney. This leads to a considerable loss of fluid from the blood, which is removed as the filtrate in the artificial kidney. Every liter of filtrate fluid that is removed in the artificial kidney, contains a large fraction of the molecules that are dissolved in the plasma, like urea, creatinine, phosphate, potassium, sodium, glucose, amino acids, water-soluble vitamins, magnesium, calcium, sodium, and other ions, and trace elements. The fraction of the molecules that passes the semipermeable membrane depends mainly on the physico-chemical characteristics of the molecules and the membrane. In order to keep the blood volume of the patient at a desired (constant) level, a substitution infusion fluid is added to the blood stream in the extracorporeal circuit, after is has passed through the artificial kidney and before it re-enters the patient's vein.
In a normal CVVH procedure, approximately 50 liters of filtrate are removed per 24 hours, and approximately the same amount of substitution infusion fluid is added into the return of blood side of the extracorporeal circuit. The substitution infusion fluid commonly used is conventional infusion fluid consisting of a physiological saline solution generally only containing about 140 mmol/L of sodium ions, 1.6 mmol/L of calcium ions, 0.75 mmol/L of magnesium ions, 36 mmol/L of bicarbonate ions, and 110 mmol/L of chloride ions. All forms of hemodialysis or hemodiafiltration therapies are characteristically different from pure hemofiltration by the use of a dialysate fluid flow along the semipermeable membrane side opposite to the blood side. The removal of molecules (clearance) in hemodialysis is dependent on the diffusion of molecules through the semipermeable membrane, while in hemofiltration the molecules are removed by pulling the plasma through the semipermeable membrane, a process that is named convection. With convection the fraction of larger molecules that passes through the semipermeable membrane is much larger than with diffusion. Therefore, all hemodialysis forms of treatment are much less effective in removing larger molecules than pure hemofiltration.
In order to prevent coagulation of the blood during hemofiltration, usually an anticoagulant is added to the blood in the extracorporeal circuit before it enters the artificial kidney. In the past, heparin or fractionated heparin was often used for this purpose. A drawback of the use of heparin, however, is that this use leads to systemic anticoagulation (i.e., anticoagulation of all blood including that within the patient), giving rise to the risk of the occurrence of serious bleeding complications, particularly in seriously ill patients.
Instead of heparin, citrate ions can be used as anticoagulant, as has been proposed for the first time by Pinnick et al., New England Journal of Medicine 1983, 308, 258–263, for hemodialysis. Citrate ions, usually added in the form of trisodium citrate, are believed to bind free calcium ions in the blood, which have a pivotal role in the coagulation cascade.
Citrate ions, added to the blood into the extracorporeal circuit before it enters the artificial kidney, are only active as an anticoagulant in the extracorporeal circuit, whereby the risk of bleeding complications due to systemic anticoagulation is avoided. When citrate ions are applied during hemodialysis forms of treatment, a calcium-and magnesium-free substitution fluid or dialysate is required. Therefore, the application of citrate ions during hemodialysis is more complex than during pure hemofiltration.
Citrate ions are mainly metabolized in skeletal muscle and liver tissue. Only in cases of severe hepatic failure combined with severe shock, or of certain (rare) metabolic diseases, the metabolism of citrate may run short, leading to too high citrate concentrations in the systemic blood circulation, which on its turn may endanger the patient. Accordingly, citrate ions are an attractive anticoagulant for use in pure hemofiltration procedures, especially for use in CVVH treatment in ICU patients.
During hemofiltration, part of the citrate ions is removed from the blood in the artificial kidney. The citrate ions that flow over into the systemic circulation of the patient, are rapidly metabolized to bicarbonate ions in skeletal muscle and liver tissue (about 2.8 molecules bicarbonate are made from 1 citrate molecule). Because trisodium citrate contains on a molar basis three times as many sodium ions as citrate ions, the sodium ions that flow over into the systemic circulation of the patient significantly increases the blood sodium concentration. As a result, hypernatremia and/or an abnormal increase in bicarbonate ions (metabolic alkalosis) may occur. Therefore, replacement of a part of the trisodium citrate by citric acid may reduce the sodium load and, by its acid component, neutralizes part of the bicarbonate generated. Accordingly, a mixture of trisodium citrate with citric acid, is a more attractive anticoagulant for use in hemofiltration procedures than trisodium citrate alone, especially for use in CVVH treatment in ICU patients.
Because citrate ions bind to positively charged metal ions like calcium, magnesium, iron, zinc, copper, and manganese, these ions are also partly removed in the artificial kidney, leading to a net removal of calcium and magnesium ions and other metal ions from the patient's blood. As a result, hypocalcemia and/or hypomagnesemia and/or shortages of other metal ions may be induced in the patient. Especially the hypocalcemia, hypomagnesemia, and/or metabolic alkalosis, may induce life-threatening complications in the patient.
The process of hemofiltration, induces a net removal of phosphate and potassium ions, trace elements, water-soluble vitamins, amino acids and of glucose in the artificial kidney. For example, when during CVVH 50 liters of plasma filtrate per day are removed, it usually contains all of the dissolved urea, creatinine, sodium, potassium, and bicarbonate, and significant amounts of the other dissolved molecules like phosphate, calcium salts, trace elements, water-soluble vitamins, amino acids, and/or glucose. This may lead to significant degrees of hypovolemia, hypophosphatemia, hypokalemia, and shortages of trace elements, water-soluble vitamins, amino acids, and/or glucose, with the risk of deteriorating the patient's condition. Especially the hypophosphatemia may also induce life-threatening complications in the patient. In order to prevent these complications from occurring, it is crucial to return an appropriate volume of substitution infusion fluid per unit of time, containing appropriate amounts of the removed molecules that are needed by the patient.